A wireless communication device, such as a mobile phone device or a smart phone, may include two or more Subscriber Identity Modules (SIMs). Each SIM may correspond to at least one subscription via one or more Radio Access Technologies (RATs). Such a wireless communication device may be a multi-SIM wireless communication device. In a Multi-SIM-Multi-Active (MSMA) wireless communication device, all SIMs may be active at the same time. In a Multi-SIM-Multi-Standby (MSMS) wireless communication device, if any one SIM is active, then the rest of the SIM(s) may be in a standby mode. The RATs may include, but are not limited to, Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA) (particularly, Evolution-Data Optimized (EVDO)), Universal Mobile Telecommunications Systems (UMTS) (particularly, Time Division Synchronous CDMA (TD-SCDMA or TDS), Wideband Code Division Multiple Access (WCDMA), Long Term Evolution (LTE), High-Speed Downlink Packet Access (HSDPA), and the like), Global System for Mobile Communications (GSM), Code Division Multiple Access 1× Radio Transmission Technology (1×), General Packet Radio Service (GPRS), Wi-Fi, Personal Communications Service (PCS), and other protocols that may be used in a wireless communications network or a data communications network.
A MSMS (e.g., a Dual-SIM-Dual-Standby (DSDS)) wireless communication device may have multiple subscriptions. In some cases, a first subscription may be utilized for Internet Protocol (IP) Multimedia Subsystem (IMS), providing IMS services such as Voice-Over-LTE (VoLTE), Short Messaging Service (SMS), Rich Communication Service (RCS), etc. A second subscription may be a Designated Data Subscription (DDS) for data services. In this configuration, IMS service may be conducted at the first subscription through packet-switched network (e.g., LTE network). If the IMS voice service is currently unavailable, the wireless communication device may use Circuit-Switch Fall Back (CSFB) to conduct voice service. With CSFB, the LTE network acts as an intermediary between the wireless communication device and a legacy circuit-switched network (e.g., 2G/3G network). When the wireless communication device is to originate a voice call, it sends a Service Request Message (SRM) to the LTE network, which transfers the wireless communication device to the circuit-switched network. When a voice call comes in, the call request first reaches the circuit-switched network, which sends paging messages to the LTE network via Serving Gateways (SGs) interface. The LTE network then forwards the paging messages to the wireless communication device. If the call is accepted, the wireless communication device sends a SRM to the LTE network, which transfers the wireless communication device to the circuit-switched network. When the call is over, the wireless communication device returns to the LTE network. As such, voice calls conducted through CSFB may ensue additional signaling and delay.